In recent years, significant development work has been done in the field of ink jet printing. One type of ink jet printing involves electrostatic pressure ink jet, wherein electrostatic ink is applied under pressure to a suitable nozzle. The ink is thus propelled from the nozzle in a stream which is caused to break up into a train of individual droplets which must be selectively charged and controllably deflected for recording or to a gutter. The droplet formation may be controlled and synchronized by a number of different methods available in the art including physical vibration of the nozzle, pressure perturbations introduced into the ink supply at the nozzle, etc. The result of applying such perturbations to the ink jet is to cause the jet stream emerging from the nozzle to break into uniform droplets at the perturbation frequency and at a predetermined distance from the tip of the nozzle. It is of utmost necessity in such systems to precisely synchronize the application of the appropriate charging signal to the ink droplet stream at the precise time of droplet formation and breakoff from the stream. Means for supplying the selected electrostatic charge to each droplet produced by the nozzle conventionally comprise a suitable charging circuit and an electrode surrounding or adjacent to the ink stream at the location where the stream begins to form such droplets. Charging signals are applied between a point of contact with the ink and the charging electrode. A drop will thus assume a charge determined by the amplitude of the particular signal on the charging electrode at the time that the drop breaks away from the jet stream. The drop thereafter passes through a fixed electric field and the amount of deflection is determined by the amplitude of the charge on the drop at the time it passes through the deflecting field. A recording surface is positioned downstream from the deflecting means such that the droplet strikes the recording surface and forms a small spot. The position of the drop on the writing surface is determined by the deflection the drop experiences, which in turn is determined by the charge from the droplet. By suitably varying the charge, the location at which the droplet strikes the recording surface may be controlled with the result that a visible, human readable, printed record may be formed upon the recording surface. U.S. Pat. No. 3,596,275 of Richard G. Sweet entitled "Fluid Droplet Recorder" discloses such a recording or printing system.
The time that the drop separates from the fluid stream emerging from the nozzle is quite critical, since the charge carried by the droplet is produced at that moment by electrostatic induction. The field established by the charging signal is maintained during drop separation, and the drop will carry a charge determined by the instantaneous value of the signal at breakoff. In order to place exact predetermined charges on individual droplets in accordance with successive video signals, it is necessary to know exactly the time of drop breakoff in relationship to the timing of the charge signal. Stated differently, the droplet breakoff time and the application of the charge signal must be very precisely synchronized. Failure to properly synchronize drop breakoff and the charging signal results in very imprecise control of the printing process with attendant degradation of the print quality.
Synchronization may also be important in the binary type electrostatic printing wherein uncharged drops are not deflected and proceed directly to impact recording medium, whereas charged drops are deflected to the gutter. U.S. Pat. No. 3,373,437 of Richard G. Sweet et al entitled "Fluid Droplet Recorder with a Plurality of Jets" discloses such a recording or printing system.
In this type of system, if synchronization is not correct such that the charging signal is in the process of either rising or falling at the time of drop breakoff, the exact charge of the drop will be some time function of the maximum charge signal rather than being fully charged. Such drops may be deflected by an amount too small to cause impact with the gutter, but instead would impact the recording medium at an unintended position.
With respect to the problem of obtaining proper synchronization between the charged signal and drop breakoff, the prior art definitely recognized the criticality of the synchronization problem and many techniques have ben proposed to test the drops for proper charging and adjust the synchronization between the charging signals and the perturbation means. The following U.S. patents are representative of the prior art: Lewis et al, U.S. Pat. No. 3,298,030; Keur et al, U.S. Pat. No. 3,465,350; Keur et al U.S. Pat. No. 3,465,351; Lovelady et al U.S. Pat. No. 3,596,276; Hill et al U.S. Pat. No. 3,769,630 (above); Julisburger et al U.S. Pat. No. 3,769,632; and Ghougasian et al, U.S. Pat. No. 3,836,912.
The Lewis et al patent describes drop synchronization using a phase shifter to ensure proper charging of drops at the correct time. The Keur et al, U.S. Pat. No. 3,465,350, describes the use of a test 33kHz. train of slightly narrowed pulses to charge drops for deflection to a test electrode, which is impacted only by fully charged drops. The detector thus supplies an output signal only when the phasing is correct. The Keur et al U.S. Pat. No. 3,465,351 describes similar charging of the drops and the implacement of a target bar so that all drops strike the bar, together with an integrated measurement of the total current given out by the drops to indicate proper or improper phasing. In both patents, the 33kHz. charging rate for the test signals is the normal charging rate for the printing video signal. The Lovelady et al patent also charges each drop of the stream to impact the gutter and directly compare the resultant gutter voltage against the reference voltage to establish whether the appropriate phase relationship exists. The Hill et al patent discloses a dual gutter arrangement for using the voltage resulting from drops impacting at either extreme of deflection for detecting whether proper phasing has been achieved. The Julisburger et al patent discloses the use of slightly narrowed selective phase charging signals for testing the phase adjustment of each of a series of drops and an induction sensing means and digital phase detection circuitry for determining whether the drops are properly synchronized. The Ghougasian et al patent is directed to a specific induction sensing means located near the charge electrode and prior to the deflection means useful for synchronization detection.
With the exception of the Keur et al U.S. Pat. No. 3,465,350 and the Ghougasian et al patents, all of the foregoing art is subject to very poor signal to noise ratios on the detected signals and, as the result, is subject to a high probability of inaccuracy, or requires an intricate array of shielding to attempt to reduce the signal to noise to usable levels. The Ghougasian et al patent simply describes an induction sensor which may be utilized with the system of the Julisburger patent. The Keur et al U.S. Pat. No. 3,465,350 patent is primarily an aiming test which may be affected by other parameters.